1. Field of the Invention
The present invention relates to a water base ink set for ink-jet recording which includes a pigment-based black ink containing a self-dispersible black pigment and a dye-based yellow ink containing a water-soluble yellow dye. More specifically, the present invention relates to a water base ink set for ink-jet recording which is suitable for the use in an ink-jet printer mounted with a piezoelectric ink-jet printing head, especially with a piezoelectric ink-jet printing head manufactured by stacking sheets each having a ink channel formed therein.
2. Description of the Related Art
As a water base ink used in the ink-jet recording system, dye-based inks containing a water-soluble dye which hardly causes deposition and is excellent in handling performance as a coloring agent have been hitherto widely used. The dye-based ink, however, has a disadvantage such that the phenomenon of the blurring, which causes the edge of an image area or a line area to become jaggy, is likely to occur because the dye-based ink is prone to irregularly spread along the surface of a recording paper when the ink permeates into the paper. As a result, it is difficult to obtain an image area or a line area with sharp edge.
In order to suppress the blurring and to obtain an image area or a line area with sharp edge, a pigment-based ink using, as a coloring agent, a pigment which disperses as minute particles in the ink has been used as the water base ink for ink-jet recording for the following reason. It is considered that, unlike the water-soluble dye dissolved in an ink solvent medium, the pigment is not dissolved in the ink solvent medium and does not behave in the same manner as the ink solvent medium. Accordingly, the pigment particles in the pigment ink are prohibited from moving (permeating) by, for example, the additives and fibers contained in the recording paper. Therefore, the amount of the pigment particles remained on the paper surface is increased.
Although the pigment-based ink has these characteristics, there is a problem associated with the pigment-based ink such that the vividness or brightness of a printed article is deteriorated because light is diffusely reflected by the pigment particles on the surface and in the inside of the paper on which the ink-jet recording has been performed. On the other hand, although the dye-based ink has the problem as described above, the dye-based ink is advantageous in obtaining printed article with vivid or bright color because the dye is dissolved in the ink at the molecular level, and thus a dye molecule absorbs light with a wavelength characteristic to the dye molecule without diffusely reflecting the light on the surface and in the inside of the paper.
In view of this situation, an ink set has been devised in U.S. Pat. No. 5,745,140 in which a pigment-based black ink using a black pigment such as carbon black as the coloring agent and a dye-based color ink using a dye as the coloring agent are used in combination. When such an ink set is used, for example, text data is printed with the pigment-based black ink and graphic data is printed with the dye-based color ink. Accordingly, it is possible to form not only letters excellent in visibility (visual. recognition performance) and with sharp edges and high contrast but also color images with vivid colors.
The pigments used in the pigment-based ink are generally classified into the following two types by the way the pigments are dispersed in the water. One type is a dispersing agent-aided dispersion pigment with which a dispersant agent such as a surfactant and a resin selected in accordance with the intended use for the ink is used. The other type is a self-dispersible pigment in which a surface oxidation treatment, a chemical bonding treatment or the like is applied to the particles of the pigment such as carbon black so that the pigment particles can be dispersed in the water by themselves. In general, the self-dispersible pigment is widely used for having the following advantages. As compared with the dispersing agent-aided dispersion pigment, the dispersion state in the self-dispersible pigment is hardly deteriorated even when co-existing with components necessary for the water base ink for ink-jet recording such as a moistening agent, a permeating agent, a surfactant and other additives, thereby realizing an increased freedom (flexibility) in the ink composition. On the other hand, the self-dispersible pigment, in which the dispersion stability relies only on the electrostatic repulsive force, has a problem such that the dispersion of the self-dispersible pigment is likely to become unstable when the pigment co-exists with salts.
Accordingly, in an ink-jet printer which is mounted with an ink set for ink-jet recording including the above-described pigment-based ink and dye-based ink, when the self-dispersible pigment which is negatively charged in the ink is used as the pigment and a general anionic dye is used as the dye, there may arise the following problem. When the pigment-based ink and the dye-based ink contact with each other or mix with each other, the negative charge of a pigment particle is canceled by the positive charge of a counter ion of the anionic dye, the electric repulsive force of the pigment is lost and the dispersion is destabilized, which results in the aggregation of pigment in some cases. In addition that the dispersion state of the pigment is destabilized due to the lowered electrostatic repulsive force as described above, the aggregation of the pigment may also occur in other cases due to a solvent shock which is caused when the pigment ink and the dye color ink contact with each other, thereby destabilizing the dispersion of the pigment.
Further, when an image is formed on the recording paper with such a water base ink set for ink-jet recording, the problem of the bleeding also occurs. The bleeding refers to such a phenomenon that the printing quality deteriorates at a portion (hereinafter referred to also as a boundary portion), in which different colors are adjacent to each other, because the inks having different colors are mixed with each other at the portion. In particular, the bleeding between a pigment-based black ink and a dye-based yellow ink is a serious problem since these inks greatly differ from each other in lightness and darkness or contrasting density.
As the attempt for solving the problem of bleeding in such a water base ink set for ink-jet recording, a method is devised in which a first electric charge (anionic or cationic) is applied to a black ink, and a second electric charge (cationic or anionic), which is opposite to the first electric charge, to a color ink to be used in combination with the black ink (U.S. Pat. No. 6,386,695 corresponding to Japanese Patent Application Laid-open No. 2002-173623). In this method, the black pigment particles contained in the black ink are intentionally made to aggregate on the paper surface when the black and color inks contact with each other, thereby momentarily preventing the pigment in the black ink from moving off a border with the color ink. As a result, a good anti-bleeding quality is obtained. Accordingly, it is expected to suppress the bleeding by applying this method to the pigment-based black ink and the dye-based yellow ink.
However, even when the bleeding can be suppressed on the surface of recording paper, the aggregation of the pigment still occurs in a nozzle head surface or the like of an ink-jet printer when the pigment-based black ink and the dye-based yellow ink make contact or mix with each other in the nozzle head surface. Accordingly, when such an ink set for ink-jet recording including the pigment-based black ink and the dye-based yellow ink as described above is applied to a generally used conventional ink-jet printer, various inconveniences are expected to occur.
For example, such a generally used conventional ink-jet printer is disclosed in U.S. Pat. No. 6,631,974 B2 corresponding to Japanese Patent Application Laid-open No. 2002-234151. This ink-jet printer has a simple maintenance structure in which nozzles for different colors in a printing head are collectively wiped. Accordingly, in this printer, the nozzle arrays, which are arranged in the printing head to discharge inks of two colors respectively, are disposed extremely close to one another. Therefore, it is impossible to divide, for each of the colors, the suction cap for sucking and purging the inks in the printing head. In this structure, the nozzles arrays for the inks of two colors such as a pigment-based black ink and a dye-based yellow ink are collectively covered with the suction cap. Therefore, the inks of two colors, which are simultaneously sucked by the aid of the suction cap, are mixed with each other in the suction cap, delivered to the suction pump through a tube, and then discharged to a drain tank by the aid of the suction pump. Therefore, when this simple maintenance system is adopted in the printer, the cost of the printer can be suppressed but the following problems arise. When the wiping is performed in this printer, the pigment-based black ink and the dye-based yellow ink are mixed with each other in the nozzle surface of the printing head, the wiper and the suction cap; and the mixed ink adheres to or firmly fixes to the nozzle surface of the printing head, the wiper and the suction cap. In addition, the pigment-based black ink and the dye-based yellow ink are mixed with each other also in the suction cap, a tube connecting respective members, the pump, and the drain tank; and the mixed ink adheres to or firmly fixes to the inside of these parts. The pigment are aggregated in the mixed ink, and the aggregated pigment causes the clog-up of nozzle of the printing head; and the aggregated pigment adheres or firmly fixes to the surrounding of nozzle, thereby damaging a repellent ink coat, which in turn causes the discharge failure and the degradation of printing quality. In addition, the aggregated pigment adheres or firmly fixes to the wiper, thereby causing non-uniform wiping operation; the aggregated pigment adheres or firmly fixes to the suction cap to lower the airtightness; the aggregated pigment adheres to and firmly fixes to the inside of the suction pump, thereby causing the operational failure of the pump or the like. As a result, the reliability of maintenance is lowered.
In order to suppress these inconveniences, it is necessary to elaborate the constitution of the printer itself in which the ink set is used. For this purpose, countermeasures are conceivable such that providing a plurality of wipers for the pigment-based black ink and the dye-based yellow ink respectively to prevent these inks from mixing with each other in the surrounding of the nozzles; separating the printing head into a head for the pigment-based black ink and a head for the dye-based yellow ink; and separating the maintenance system into a maintenance system for the pigment-based black ink and a maintenance system for the dye-based yellow ink. However, these countermeasures to elaborate the structure of the printer has problems such that the structure of the printer becomes complicated and the cost for manufacturing the printer is increased. Therefore, it has been practically difficult to provide a water base ink set for ink-jet recording including a pigment-based black ink and a dye-based yellow ink in a generally used conventional ink-jet printer of the low-cost type.
Further, there is a demand for a dye-based yellow ink, on the premise to be used in combination with a pigment-based black ink, namely the dye-based yellow ink capable of expressing a tint of complex yellow such as a greenish yellow, an orangey yellow, a reddish yellow or the like, rather than a tint of a simple yellow. The use of a single yellow dye, however, is not capable of achieving this objective under the present circumstances.
In connection with this demand, as the yellow dye for yellow ink having a satisfactory color reproduction property, it has been devised to use Color Index (hereinafter abbreviated as “C. I.”) Direct Yellow 86 and C.I. Direct Yellow 132 in combination (U.S. Pat. No. 5,560,771 corresponding to Japanese Patent No. 3089665). There is, however, a problem that the aggregation of pigment-based black ink cannot be suppressed, even when it is possible to obtain a yellow ink having a satisfactory yellow coloring with complex yellow tint, rather than the tint of simple, by using C. I. Direct Yellow 86 and C.I. Direct Yellow 132 in combination.